There's physical illness that afflicts several of the characters, and the author uses it to convey a sense of psychological trouble. For example, Pop has a case of athlete's foot of the hands (???) that just won't go away no matter what he tries. After the Knights start to win and his spirits improve,
His hands healed and so did his heart, for even during the tensest struggle he looked the picture of contentment. (4.23)
Memo also has a mysterious illness. She tells Roy that her breast is "sick." We never find out what the sickness is, but it hints at how dangerous Memo's sexuality is to Roy.
Roy himself has a couple bouts of illness. Just before his last at-bat, Pop begs Roy to win the game. He's burdened by the fact that he's agreed to strike out. It affects him physically:
With a heavy heart, Roy pulled up to the plate. He had shooting pains in back muscles that had never bothered him before and a crick in his neck. He couldn't comfortably straighten up and the weight of Wonderboy crouched him further. (10.42)
Malamud obviously knew about the mind-body connection even before Dr. Oz.
Then there's the whopper of a stomach problem that lands him in the hospital after eating Memo's food, and it's not hard to see the connection between his illness and his being morally compromised by giving in to Memo and taking the Judge's offer.